1. Field of the Invention
The present invention relates to a position measuring system for accurately measuring the distances among a plurality of GPS (Global Positioning System) receiving units using radio waves of position measured information received from satellites.
2. Description of the Related Art
Position information systems that inform their users of their positions using GPS have become common. In the GPS, spectrum spread signals of ephemeris data and almanac data are received from several to a dozen satellites. Corresponding to the received data and signals, position information (latitude and longitude) is obtained. Almanac data is used to determine radio waves of satellites that a GPS receiving unit can acquire. Ephemeris data is accurate position information of a satellite and is used to obtain the distance between the GPS receiving unit and a satellite.
The GPS receiving unit receives data from for example three satellites and calculates pseudo-distances to the satellites corresponding to time periods after data is transmitted from the satellites until the data is received. In addition, the GPS receiving unit receives data from another (fourth) satellite, corrects errors of the pseudo-distances, and obtains the current position of the GPS receiving unit.
When the distances among a plurality of GPS receiving units are calculated using such a system, the measuring accuracy is important. Examples of deterioration factors of the measuring accuracy are the performance of the GPS receiving unit and spatial delays of radio waves received from satellites. In addition, since the system was developed for military use, as another deterioration factor, there is intentional deterioration of position measured information.
Such deterioration factors cause position measurement errors of as large as several 10 meters. In addition, the magnitudes and patterns of position measurement errors depend on the types of transmitting units. Thus, the receiving unit side cannot predict position measurement errors.
To solve such a problem, a base station whose coordinates are known receives position measured information from a transmitting unit. By comparing the measured result with the coordinates of the base station, a position measurement error on a transmitting unit side is calculated. FIG. 1 shows a method for measuring a position measurement error on a base station side. Referring to FIG. 1, a base station E has a GPS receiving unit. The coordinates of the position of the base station E are known. The base station E receives radio waves from satellites A, B, C, and D. The distance between each of the satellites A, B, C, and D and the base station E is obtained using the received radio waves. By comparing the measured values with the coordinates of the position of the base station E, errors .DELTA.A, .DELTA.B, .DELTA.C, and .DELTA.D of the satellites A, B, C, and D are obtained. When the position of the base station E is actually measured, the errors .DELTA.A, .DELTA.B, .DELTA.C, and .DELTA.D are added to the position measured information of the satellites A, B, C, and D, respectively.
In the GPS, a system that calculates errors and provides compensation information using FM radio waves has been practically used. With such a system, the user can obtain accurate position measured information.
However, in such a system, when the user's position is out of the service area of the system, he or she cannot use the service.
When the user uses the service, he or she requires complicated hardware. Occasionally, the user should pay subscription fee for the service. Thus, the user's cost becomes high. Moreover, other position calculation radio wave transmission systems other than GPS do not have such a service.
On the other hand, when the relative distances among a plurality of receiving units are required, the accuracies of absolute position information of the receiving units are not important as long as the accuracies of the distances among the receiving units satisfy a predetermined level. Thus, the conventional systems cannot satisfy such requirements.
In addition, to stably accomplish such a function, the accuracy should be maintained in a particular level anywhere. However, in the conventional systems, since satellites used for measuring the user's position are changed from time to time, it is difficult to maintain the particular level of accuracy.